1. Field of the Invention
The present invention relates to a device for protecting a power semiconductor device such as an insulated gate bipolar transistor (IGBT) or metal oxide semiconductor field effect transistor (MOSFET) against a short circuit.
2. Description of the Related Art
Conventionally, the protection of a power semiconductor device such as an IGBT from a short circuit is made as follows. First, a short-circuit current is detected and a detection signal thereof is transmitted to a control circuit via an insulated signal coupling means such as a photocoupler. The control circuit drives a driving circuit to supply a turn-off signal to the power semiconductor device, thus protecting the power semiconductor device from the short circuit.
FIG. 1 shows an example of a conventional protective device against short circuit using a current detector 2 as a means for detecting a short-circuit current. When the current detector 2 detects the short-circuit current, a detection signal thereof is supplied to a control circuit 6 via an insulated signal coupling circuit 4 such as a photocoupler. The control circuit 6 causes a driving circuit 8 to output a turn-off signal so as to turn off an N-channel IGBT 9.
FIG. 2 shows an example of a conventional protective device against short circuit using a resistor 3 having a large power capacity instead of the current detector 2 as a means for detecting a short-circuit current. The resistor 3 detects a short-circuit current based on a potential difference between the voltages at both ends thereof and the detection signal is supplied to the control circuit 6 via the insulated signal coupling circuit 4. The control circuit 6 causes the driving circuit 8 to output a turn-off signal so as to 10 turn off the IGBT 9.
In the above conventional cases, it is necessary to use the current detector 2 for detecting a short-circuit current or the resistor 3 having a large power capacity and it is necessary to provide a relatively large space to dispose them, thereby increasing the size of the device. Further, use of the current detector 2 or resistor 3 increases the cost of the device.
Further, since the detection signal of the short-circuit current is supplied to the control circuit 6 via the insulated signal coupling circuit 4, it takes a relatively long delay time from the time of generation (detection) of the short circuit until a turn-off signal is actually transmitted to the IGBT 9, and there occurs a possibility that energy of an amount larger than the breakdown amount of the IGBT 9 by the short-circuit current is applied to the IGBT 9 in the above delay time to break down the IGBT 9.
Further, the short-circuit current is several times to several tens of times larger than a current flowing in the normal operation. Therefore, di/dt of the short-circuit current at the time of turn-off is several times to several tens of times larger than di/dt of the current at the time of turn-off in the normal operation. As a result, an excessively high voltage called a spike voltage occurs at the output terminal of the IGBT 9, causing a possibility that the IGBT 9 may be broken down.
As described above, the conventional protective device for protecting the power semiconductor device against short circuit has the following defects.
(1) It is necessary to provide a large space in order to dispose a short-circuit current detection means.
(2) Use of the short-circuit current detection means increases the cost.
(3) Since a short-circuit detection signal is fed back to the driving circuit via the insulated signal coupling means, delay time occurs in the operation to delay generation of a turn-off signal used for protecting the power semiconductor device, thereby permitting a short-circuit current to flow in the power semiconductor device for a long period of time and breaking down the semiconductor device.
(4) A high voltage is output at the output terminal of the semiconductor device by di/dt of the short-circuit current when the semiconductor device is turned off after the detection of the short-circuit current, thereby breaking down the semiconductor device.